Objective:

A new approach to mitigating nutrient loading called a permeable reactive barrier has been installed at Waquoit Bay, MA. This barrier intercepts ground water and provides a carbon source for an increased population of denitrifying bacteria. These bacteria scrub ground water of the excess nitrate that fuels eutrophication. However, the barriers also support increased populations of sulfate-reducing bacteria, which have the ability to methylate the large amounts of mercury coming in through the ground water. Only geochemical studies have been conducted on the barrier so far, so this study aims to characterize the denitrifying bacteria in the barrier to determine how populations differ from a control beach. It also aims to measure the amount of toxic methylmercury entering the bay as a result of the barrier.

Approach:

To determine if denitrifying bacteria in the barrier are distinct from denitrifying bacteria from control sites, the study will build clone libraries from ground water at both sites to look at the genetic diversity and phylogeny of the bacteria. To determine mercury concentrations, the study will measure mercury levels in the ground water throughout three seasons (summer, fall and spring because microbial activity is probably very low during the winter). Finally, the study will measure mercury concentrations in benthic infauna once a year (at the end of summer after the period of highest microbial activity) to see whether toxic methylmercury is being bioaccumulated in the food web. If this is the case, the study also will characterize the genetic variability of sulfate-reducing bacteria.

Expected Results:

It is expected that microbial denitrifying bacterial communities in the
barrier will be distinct from those at control sites due to the high amount
of degradable carbon and the unique redox conditions present within
the barrier. It also is expected that toxic methylmercury concentrations
will be elevated downgradient from the barrier relative to control sites
because of the presence of sulfate-reducing bacteria (indicated by the
production of hydrogen sulfide). If they are methylating even one-tenth
of the mercury coming in from ground water, it might be enough to accumulate
in fish tissue.

Potential to Further Environmental/Human
Health Protection
Methylmercury is the most toxic form of mercury in terms of bioaccumulation.
Very small concentrations in the water column are enough to
biomagnify to fish tissue, so if the barriers are contributing a significant
amount of methylmercury, the Town of Falmouth cannot continue with
its plan to install a 300-meter long test barrier despite its efficiency at
mitigating eutrophication.

Supplemental Keywords:

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.